Having no gaps means nice small welds and no grinding.
All plates were sprayed on the backside with weld through primer then were TIG welded. Although it takes much longer to Tig weld than MIG, the finished item is much nicer (and you don't get covered in weld hot weld splatter).

With all the mounts/brackets prepared, i then went onto puddle weld them back
onto the car. All welds were blended so that it still maintained the original factory look.

Made up these little spacers,

All welded,

Next was to do the three steps of the POR15, Degrease, MetalPrep and then the Rust Preventive Paint. I ended up not just doing the corroded areas but everything that was bare metal.

I highly recommend the POR15, i was a bit worried about washing off the MetalPrep with water as it was directly to bare metal and i didn't want to introduce any more rust. But it left an 'etched' zinc finish that couldn't be welded over, two coats of the Rust Preventive paint turned out to be so good that it was a struggle to get a keyed finish so that it could be painted over!

That looks great! How does the swap affect the handling on something like these? I'd imagine the weight distribution would be affected?

Thanks, To be honest i don't know until i drive it. The overall weight of the compact isn't much different to the coupe, although the S54 is Heavier than the M54 ive taken out of it so the front end will have more weight over it.

Next was onto joining all the layers of the boot floor together and bracing the subfame.
There seems to be many variations of how people do this i.e plates, solid bars, box sections, bonding and cutting perfectly good panels up to gain access.

Here's how i done mine,

6mm Drill bit extended and a aluminium spacer turned up, this then goes from the underside into the mounting holes of the subframe bolts. The spacer slides over the drill bit keeping it central and avoids the internal threads getting damaged while being drilled.

After drilling the 6mm hole through each layer of the floor, i then went inside the car and opened the hole up in the first layer of boot floor with a hole saw.

There is a raised bead inside the next layer of boot floor, this is what the threaded section is mounted to. The raised bead is there for structural reasons and shouldn't be flattened to close the cavity gap,

Next I turned two small spacers, one for each side.

With a smaller hole cutter matching the the size of the spacer diameter, I carefully cut into the raised beaded layer and removed it leaving the top of the threaded section on show. The spacer was then fully TIG welded into the hole that was previously cut picking up on the factory MIG welds. This closed any gap and gave a good mounting point for the next stage.